The invention relates to a method and a system for controlling the charging of a vehicle battery.
Electrical on-board power supplies of motor vehicles normally have a vehicle battery and a generator which is connected parallel thereto and continuously generates electric power when the engine is running. As a rule, the generator power is sufficient for supplying all consuming devices connected to the on-board power supply. The vehicle battery is therefore usually used only as an energy accumulator for the starting operation and for the short-term supply of the electrical consuming devices, for example, in the event of a brief engine stoppage. Particularly after a cold start, the vehicle battery should expediently be supplied with electric power by the generator until the vehicle battery is completely charged. Even afterwards, it can still be charged further at a low charging current in order to maintain the charged condition.
In the case of vehicles with internal-combustion engines, the vehicle interior is normally heated by using engine waste heat. However, in the warm-up phase after a cold start, internal-combustion engines emit only little heat. This applies particularly to high-efficiency internal-combustion engines, such as modern diesel engines.
It has therefore been suggested to operate an electrical auxiliary heater during the warm-up phase, which is supplied with power by an electrical on-board power supply. German Patent Document DE 197 28 589 C1 describes a system with such an auxiliary heater. In order to utilize the generator well, a so-called dynamo field signal (DF signal) is tapped which controls an exciting current of the generator and is a measurement for the utilization of the generator, and the generator utilization is regulated to a defined value by adapting the electric resistance of the auxiliary heater. During the operation of the auxiliary heater, the vehicle battery is permanently connected to the generator.
This approach has the disadvantage that the power offered to the auxiliary heater depends on the charging condition of the vehicle battery and on the power requirement of the electrical consuming devices on the on-board power supply, and an undersupply and/or heating capacity fluctuations may therefore occur which are noticed by the vehicle occupants. This particularly applies to systems with vehicle batteries having a high storage capacity which can be charged with very high charging currents, for example, currents of more than 150 A. In order to ensure a supplying of the auxiliary heater with electric power in such cases, generators have been used in practice which have a particularly high nominal generator power. However, such generators are heavy, are expensive with respect to their manufacturing and require a lot of space.
It is an object of the present invention to provide a method and a system which permit a good utilization of the generator, prevent a discharge of the vehicle battery, and nevertheless, even at normal nominal generator power and in warm-up phases, permit a supplying of an auxiliary consuming device with electric power which is as uniform as possible. The auxiliary consuming device is particularly an auxiliary electrical heater but may also be another consuming device which is provided in addition to other electrical consuming devices and which does not have to be supplied with electric power in a permanent manner and/or with priority. The generator supplies the additional consuming device with electric power only when the power requirement of the on-board power supply has been met.
An essential idea of the present invention is the adjusting of a charging current, by which the generator charges the vehicle battery, in such a manner that the charging current is lower than the maximally possible charging current while the generator power and the power requirement of the on-board power supply are the same. This adjustment will be made when it is determined that the charging current is greater than or equal to a limit value, when a power demand of the additional consuming device exists, and when the generator power permits a supplying of the additional consuming device. The charging current is preferably adjusted such that it does not fall below the limit value. This ensures a charging of the vehicle battery if required, particularly in a low charging condition of the vehicle battery.
One possibility of adjusting the charging current consists of providing a control element in a current path from the generator to the vehicle battery and of operating this control element such that the charging current is adjusted to a value lower than maximally possible.
The charging current is preferably indirectly adjusted in that an electric resistance of the additional consuming device is adjusted. German Patent Document DE 197 28 589 C1 describes two suitable variants which may also be combined. According to the first variant, the additional consuming device, in this case, the auxiliary heater, has a plurality of parallel-connected heating elements which can be switched on and off individually. According to the second variant, the auxiliary heater has a power transistor, such as a MOSFET (metal oxide semiconductor field effect transistor) whose heating resistance can be adjusted by controlling a control input of the transistor. As a result, on the one hand, a sufficient charging of the vehicle battery can be achieved and, on the other hand, because of the reduction of the charging current, in comparison to the maximally possible charging current, a more uniform supplying of the additional consuming device can be achieved. Furthermore, a generator with a lower nominal power can be used than when a power reserve has to be held for the event of high battery charging currents. On the whole, the generator is therefore better utilized.
In a further development, the utilization of the generator is still increased in that the charging current is increased when the power demand of the additional consuming device has been met to a certain degree, in particular, is fully met.
In an embodiment of the invention, the charging current is controlled, at least under the above-mentioned conditions, to the limit value. This means that it is attempted to achieve that the charging current is neither lower nor higher than the limit value. For example, because of dead times and/or inertias in the control circuit, the charging current may, however, deviate from the limit value. For example, the adjusting of the charging current, as described above in connection with German Patent Document DE 197 28 589 C1, takes place by adjusting the electric resistance of the additional consuming device.
However, the charging current can also be regulated to a higher value when and/or as long as the generator power is sufficient for this purpose and, in addition, also allows the supplying of the auxiliary consuming device.
Preferably, the additional consuming device will not be supplied or no longer be supplied with electric power by the generator when the charging current is lower than the limit value and/or when the charging current falls below the limit value not only for a short time or not only because of a controlling of the charging current.
In a further development, a charging condition of the vehicle battery is detected and it is permitted that the charging current falls below the limit value and/or that the vehicle battery is discharged when a supply demand of the additional consuming device exists and when the charging condition reaches a high level, particularly, when a measurement for the charging condition is higher than or at least equal to a certain minimal value. In this case, when the charging condition is not critical, the additional consuming device can therefore nevertheless be supplied by the generator, although the generator power may, under certain circumstances, not be sufficient for simultaneously charging the vehicle battery by means of a charging current which is higher than the limit value. As a result, a still better coverage of the supply demand of the additional consuming device can be achieved and/or a generator with a lower nominal power can be used.
Preferably, the limit value is defined. In particular, the limit value is a function of at least one variable, perhaps of the vehicle battery temperature and/or the battery charging condition. For example, in the case of a higher energy amount accumulated in the battery, a lower limit value is defined. It is also possible to change the limit value as a function of the momentary generator power, in which case there is preferably no falling below a defined minimal limit value. However, if the battery is fully charged, there may be a falling below this minimal limit value.
It is suggested to analyze a demand signal which reflects the electrical supply demand of the additional consuming device, and to carry out the control of the charging of the vehicle battery described here as a function of the analyzing result. By means of the demand signal, a supplying of the additional consuming device with electric power is demanded.
In particular, the charging current is permitted to assume the highest possible value permitted by the momentary generator power, if there is no supply demand of the additional consuming device. As an alternative or in addition, the limit value of the charging current is adapted when the supply demand changes.
A generator is any type of device which generates electric power. The generator may, for example, be a conventional electromagnetic generator driven by an engine or may be a fuel cell. Also in the case of a fuel cell, a load condition or degree of utilization of the generator can be determined which can be analyzed and/or be used like the DF signal of a conventional generator.